
Until the new socket PC fit nicely, but loosely at the level where I wanted it:

Then I laid down a little "pad" of Magic Sculp epoxy putty for the new PC to press into:

And after the PC was mounted at the right depth and angle, I filled in the surrounding area with more Magic Sculp using clay sculpting tools to press and work it smoothly into place:

These are the "ankle bones" that connect the top of the foot to the bottom of the leg:

So now the ankles will be essentially double jointed.
Look at these videos for a more well-rounded view of all the action (ignore the sound, please):
Once again the main idea here is that test fitting is of the utmost importance in Garage Kit building. Even more so if you want to do modifications.